The present invention relates essentially to a process for heat treatment of steel, such as heating prior to tempering, annealing, cementation or case-hardening (carburization or carbo-nitriding) performed within a furnace in the presence of an atmosphere under constant flow obtained by mixing a carrier gas including nitrogen and if appropriate hydrogen, with an active gas formed by a hydrocarbon with a volumetric proportion of hydrocarbon comprised between 0.2 and 30% of the mixture.
The application of this known process may raise particular difficulties, especially in the case of case-hardening workpieces comprising deep bores or else of workpieces of complex shape. Small deposits of soot deleterious to the quality of the finished product may actually be produced at the bottom of these bores or of other hollow sections which are difficult to reach.
An object of the present invention is to avert or minimise the aforesaid difficulties and to allow a case hardening action which is homogenous and free of soot, even on workpieces reputed to be difficult.
The experiments performed in seeking this result rendered it possible to find that the function of carbon monoxide is of prime importance in the transfer of carbon atoms from the atmosphere to the metal. As a matter of fact, the carbon monoxide renders it possible to perform a case-hardening action on the surface of the metal workpiece by a double-layer effect. The carbon monoxide is absorbed in the form of a metallic carbon-metal composite, rendering it possible to cover the entire surface, even in the least accessible parts, of the workpieces reputed to be difficult, such as those comprising deep bores, or in the cavities of workpieces of complex shape. The hydrocarbon present in the atmosphere may then form a double layer by a bond with the carbon monoxide radicals absorbed. This results in a continuous passage of the carbon atoms thus carried by the double layer, which allows of homogenous case-hardening throughout the parts of the workpieces treated.
Experience has also shown that it is important to eliminate any trace of residual oxygen from the atmosphere, so as to prevent a destabilisation of the aforesaid double layer. As a matter of fact, the oxygen molecules cause the forming around them of nucleii of hydrocarbons in the gaseous phase and by virtue of this fact prevent the double layer formed at the metal surface from being supplied with hydrocarbon, which is shown by areas insufficiently rich in carbon on the workpieces, that is to say heterogeneities in case-hardening
It is known moreover that standard industrial nitrogen contains appreciable quantities (apt to reach 2%) of oxygen, carbon dioxide and steam, and experience has also shown that it is this oxygen fed in directly by standard industrial nitrogen, or originating from the decomposition of the carbon dioxide or steam it carries, which destabilises the double layer described above.